The term data signal is intended to mean any signal carrying data or information, including telecommunications data.
A matched filter is used to detect a signal e.g. in telecommunications. The impulse response of a realizable matched filter is a signal, which is scaled, delayed and inverted in time. At the output of the filter matched to the signal we obtain the maximum level when the signal is completely received.
The output waveform of the filter is proportional to the autocorrelation function of the signal itself. In an Additive Gaussian White Noise (AGWN) channel the matched filter maximizes the momentary signal to noise ratio and minimizes the error probability of the transmission system. If also interference is added to the signal in the transmission channel the receiver will not anymore operate in an optimal way, and the performance will be reduced.
A matched filter is also used in spread spectrum telecommunication systems. Then it can be used for the detection of a data signal and also i.e. in the search phase for the code synchronization of the receiver, so that the filter impulse response is a time inverted, scaled and delayed version of the distribution code or part of it.
The prior art is described below with reference to FIGS. 1 to 3 of the accompanying drawings, in which:
FIG. 1 shows a code synchronization system, selected as an example, in accordance with the prior art;
FIG. 2 shows the output signal of the prior art matched filter in FIG. 1; and
FIG. 3 shows the signal provided by the threshold comparison block.
FIG. 1 shows a prior art code synchronization system. The code synchronization system comprises a filter 1 matched to the spreading code, a threshold comparison means 2, and means 3 for activation of the monitoring.
The maximum value is obtained at the filter 1 output when the spreading code or a part of it is received, and it can synchronize the code generator or the receiver after the threshold comparison.
FIG. 2 shows the output signal of a prior art matched filter in an exemplary case. The exemplary case is ideal and the code contains L chips and the chip period is T seconds. The output signal 4 is now an ideal autocorrelation function containing impulses having a height of L times the level of the received signal, the repeating period being the code length L.times.T seconds.
FIG. 3 shows the signal provided by the threshold comparison block. The signal 5 of the threshold comparison block 2 has the value "1" when the output signal 4 of the matched filter exceeds the threshold level.
For example a deterministic error could be disastrous, because remnants of the interference could be evident at the output of the filter 1. Then the synchronization ability decreases, preferably if the code's autocorrelation function form is not ideal and contains side lobes, which in combination with the interference response could cause the threshold level to be exceeded. The matched filter 1 certainly attenuates the interference, but for instance the attenuation of a continuous interference at the center frequency of the filter decreases when the imbalance of the spreading code increases, or in a binary case, when the difference between the number of ones and zeroes increases.
Usually we try to remove the interference by different attenuation or correction algorithms. These methods are often complicated and require computing time in order to estimate the characteristics of the interference or the state of the channel.